Head-up display (HUD) systems are currently used in aircraft to provide pilots with essential information superimposed onto their forward field of view through the aircraft windshield. The information displayed is typically data or symbolic images indicative of flight conditions, such as the operating condition of the aircraft, environmental information, or guidance information. HUD systems are also being designed for use in automobiles and other vehicles.
In one type of aircraft HUD system, a light source emits a colored image carried by multiple wavelengths of light in response to signals generated by an image signal controller. The light rays carrying the colored image propagate through a relay lens to create an intermediate image. A wavelength selective combiner reflects the intermediate image toward a pilot, who views the image as it is superimposed on an outside world scene in the same field of view. The combiner is constructed to have multiple optical powers that correct for longitudinal color aberrations introduced by monochromatic relay lens elements and thereby presents to the pilot a correctly focused multi-colored final virtual image at or near optical infinity. The design details of a multi-color HUD system are described in U.S. Pat. No. 5,710,668, which is assigned to the assignee of this patent application.
Guidance information displayed on a HUD combiner for observation by a pilot frequently includes symbology that represents position and attitude guidance for the aircraft during flight. One example is a flare anticipation cue implemented in a Flight Dynamics Head-Up Guidance System that is certified by the Federal Aviation Administration for use on Boeing 737-300 aircraft. The flare anticipation cue alerts the pilot several seconds before the aircraft reaches a flare initiation height and indicates to the pilot the pitch up rate required at the initial part of the flare. Another example is a turn anticipation cue that is described in copending International Patent Application No. PCT/US98/14177, which is assigned to the assignee of this application. The turn anticipation cue provides guidance information to a pilot during low visibility ground weather conditions after the aircraft has touched down so that the pilot can safely taxi the aircraft to the airport terminal.
There are several common factors contributing to the likelihood of aircraft tail strike that necessitate pilot awareness to tail strike during takeoff or approach to landing. With respect to takeoff, the most common contributing factor to aircraft tail strike incidents is an improperly set stabilizer in the trailing edge up direction. This is caused by pilot setting error or incorrect loading information and results in lighter than expected control forces during the rotation. If the stabilizer position is not checked, aircraft over-rotation can occur. A second factor is rotating the aircraft prior to achievement of actual V.sub.ROT speed, which is the speed at which the aircraft nose is rotated. A third factor is excessive pitch rate (approximately 5 degrees per second) during aircraft rotation caused by pilot setting errors or pilot exuberance. For large transport aircraft, nominal lift-off pitch attitude is, for example, approximately 8 degrees, with a recommended rotation rate at approximately 2.5 to 3 degrees per second. For fully compressed landing gear, nominal tail strike angles are about 12 to 13 degrees, depending upon aircraft height above the runway. A final factor contributing to aircraft tail strike during takeoff is early use of flight director or climb out pitch reference.
With respect to approach to landing, the most common contributing factor to aircraft tail strike incidents stems from the pilot's flying an unstabilized approach below 500 feet. A second contributing factor is the use of pitch trim during a flare maneuver, resulting in unexpectedly light control forces for flare initiation. A third factor is an increasing tail wind resulting in increased aircraft sink rate while already at low thrust settings. This condition further results in an attempt by a pilot to salvage a hard landing by executing a rapid pitch-up maneuver. A final factor contributing to aircraft tail strike during the landing phase is a late go-around procedure using the flight director pitch reference for desired pitch attitudes before aircraft acceleration.
It would be desirable, therefore, to provide for display guidance information that can warn a pilot of a possible impending incident in which the aircraft tail is about to strike the ground during takeoff or approach to landing.